A method to isolate mutants with derepressed expression of cytochrome oxidases and better symbiotic performance is presented. A mutant of Rhizobium etli, CFN030, isolated by its azide-resistant phenotype, was obtained by transposon Tn5-mob mutagenesis. This mutant has a derepressed expression of cytochrome aa(3), higher respiratory activities when cultured microaerobically and an improved symbiotic nitrogen fixation capacity. This phenotype was similar to the previously described mutant CFN037, which was isolated by its increased capacity to oxidize N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) [Soberon M et al. (1990) J Bacteriol 172:1676-1680]. We show here that although both mutants have a similar symbiotic phenotype, they are affected in different genes. Strain CFN030 has the Tn5 inserted in the chromosome while in strain CFN037 the transposon was located in plasmid b. Cytochrome spectral analysis of both mutant strains in the post-exponential phase of growth, showed the expression of an additional terminal oxidase (cbb(3)) that is not expressed in the wild-type strain.